1. Field of the Invention
This invention is directed to a method of forming glasses, particularly those which are relatively arsenic-free, in manufacturing systems which employ especially platinum or molybdenum. The invention is particularly useful for forming high melting or high strain point glasses, e.g. such as are used for glass substrates for flat panel display devices, without having to use such arsenic-containing materials.
2. Technical Background
Liquid crystal displays (LCDs) are passive flat panel displays which depend upon external sources of light for illumination. They are manufactured as segmented displays or in one of two basic configurations. The substrate needs (other than being transparent and capable of withstanding the chemical conditions to which it is exposed during display processing) of the two matrix types vary. The first type is intrinsic matrix addressed, relying upon the threshold properties of the liquid crystal material. The second is extrinsic matrix or active matrix (AM) addressed, in which an array of diodes, metal-insulator-metal (MIM) devices, or thin film transistors (TFTs) supplies an electronic switch to each pixel. In both cases, two sheets of glass form the structure of the display. The separation between the two sheets is the critical gap dimension, of the order of 5-10 μm.
Intrinsically addressed LCDs are fabricated using metal deposition techniques, typically at temperatures ≦350° C., followed by standard metal etching procedures. As a result, the substrate requirements therefor are often the same as those for segmented displays. Soda-lime-silica glass with a barrier layer has proven to be adequate for most needs. A high performance version of intrinsically addressed LCDs, the super twisted nematic (STN) type, has an added requirement of extremely precise flatness for the purpose of holding the gap dimensions uniform. Because of that requirement, soda-lime-silica glass made using the float glass manufacturing process must be polished. Such polishing processes are expensive and time consuming, and generate a large amount of glass particles which have the potential to negatively impact further processing of the glass sheets. Alternatively, glass can be formed using a process which does not require polishing, e.g. fusion downdraw.
Extrinsically addressed LCD's can be further subdivided depending upon the nature of the electrical switch located at each optical element (subpixel). Two of the most popular types of extrinsically (or active matrix, AMLCD) addressed LCD's are those based on either amorphous (α-Si) or polycrystalline (poly-Si) silicon thin film transistors (TFT's).
Many of the glasses manufactured for flat panel display applications, particularly those which are formed by downdraw processes (e.g., the fusion or slot draw processes), are melted or formed using manufacturing equipment comprised of refractory metals, e.g. platinum or platinum alloys. This is particularly true in the fining and conditioning sections of the process, where refractory metals are employed in order to minimize the creation of compositional inhomogeneities and gaseous inclusions caused by contact of the glass with oxide refractory materials. In addition, many of these manufacturing processes employ arsenic as a fining agent. This is because arsenic is among the highest temperature fining agents known, meaning that, when added to the molten glass bath, it allows for O2 release from the glass melt even at high melting temperatures (e.g. above 1450° C.). This high temperature O2 release (which aids in the removal of bubbles during the melting and fining stages of glass production), coupled with a strong tendency for O2 absorption at lower conditioning temperatures (which aids in the collapse of any residual gaseous inclusions in the glass), results in a glass product essentially free of gaseous inclusions. Other fining agents typically melt and release their oxygen far too early when added as fining agents to high melting temperature glasses and reabsorb O2 too late during the conditioning process, thereby disabling their fining and oxygen re-absorption abilities.
From an environmental point of view, it would be desirable to find alternative methods of making such high melting point and strain point glasses without having to employ arsenic as a fining agent. It would be particularly desirable to find methods for making such glasses via downdraw (especially fusion-like) processes. Unfortunately, previous efforts at doing so have been hindered by the production of unacceptable amounts of bubbles (seeds) in the glass. This has been a particular problem with glasses which employ refractory metals such as platinum or platinum-containing alloys in their molten glass delivery systems. This is because such metals (e.g. platinum and molybdenum) can cause an electrochemical reaction to occur with the glass which results in bubble formation at the glass/metal interface, e.g. where the glass contacts the platinum. This bubble formation in the glass/metal contact region is referred to herein as hydrogen permeation blistering.